The same problem frequently occurs with me too. In fact, despite signal strength is shown as full on status bar, error message comes "no network" and it is true because in this situation, SIM name is being shown as "Card 1" instead of "Jio-4G". More over APN detail also vanishes. This only get reset if I reboot the phone or disable and re-enable the SIM. The problem is very annoying and supercedes all the positives of Oreo update. Please note that Wi-Fi calling is already disabled.